1. Field of the Invention
The invention relates to a process for calcining an ore-based material, in which process:
the material is passed through a precalcination device equipped with at least one fuel injector supplied with at least one fuel so as to form a fuel-injection zone at the outlet of the fuel injector and supplied with oxidizing agent by the products of combustion from a rotary kiln located downstream of the precalcination device with respect to the direction in which the material flows, then PA1 the at least partially calcined material is passed into the rotary kiln which, at its downstream end, is equipped with a primary combustion unit. PA1 a precalcination device into which the material is introduced and in which drying-out, if necessary, then heating and some of the calcination of the material are carried out, and PA1 an inclined rotary kiln into which the partially calcined material is introduced and in which calcination is completed, followed by the clinkering reaction. PA1 60% to 99% of the stoichiometric amount of oxygen needed for the combustion of the fuel are provided by the products of combustion from the rotary kiln; PA1 the oxygen concentration by volume of the products of combustion from the rotary kiln is greater than or equal to 1%; PA1 the oxygen-rich fluid is a mixture of some of the products of combustion and a gas containing at least about 20% oxygen; PA1 some of the products of combustion are drawn off and air or oxygen-enriched air and/or industrially pure oxygen with a concentration higher than about 88% is mixed with them; PA1 the adiabatic temperature of the flame produced at the outlet of the fuel injector is higher than 1000.degree. C.; PA1 the adiabatic temperature of the flame produced at the outlet of the fuel injector is higher than 1250.degree. C.; PA1 the fuel with which the fuel injector is supplied is a low-quality fuel; PA1 said oxygen-rich fluid is injected using an oxygen-rich-fluid injector which is distinct from the fuel injector; PA1 the distance between the outlet of said oxygen-rich-fluid injector and the outlet of the fuel injector is less than about 50 times the interior width of the oxygen-rich-fluid injector; PA1 an oxygen-rich fluid is injected toward the fuel-injection zone of the fuel injector; PA1 the fuel is injected using the fuel injector and the oxygen-rich fluid is injected at an angle of convergence of less than 25.degree.; PA1 the precalcination device comprises at least two fuel injectors which are supplied respectively with at least one fuel to form a fuel-injection zone at its outlet and at least one oxygen-rich fluid with an oxygen concentration by volume that is higher than that of the products of combustion from the rotary kiln is injected near to the fuel-injection zones of said at least two fuel injectors; PA1 at least one oxygen-rich fluid with an oxygen concentration by volume higher than that of the products of combustion from the rotary kiln is injected by means of a fuel injector belonging to the precalcination device; PA1 said oxygen-rich fluid is used as a carrier fluid for carrying a fuel into said fuel injector; PA1 an oxygen-rich fluid is oxygen with a purity greater than 90% that is passed through said fuel injector via an oxygen-specific passage; PA1 a high-quality fuel is introduced through a passage of said fuel injector near to an oxygen-specific passage so as to form a pilot flame at the outlet of said fuel injector; PA1 at least one or each oxygen-rich fluid is oxygen-enriched air; PA1 at least one oxygen-rich fluid has an oxygen concentration higher than 90%; PA1 an oxygen-rich fluid with an oxygen concentration higher than that of air is injected into a fuel-injection zone of the primary combustion unit of the rotary kiln (FIGS. 2 and 4 to 7); PA1 the oxygen-rich fluid is introduced inside a fuel injector belonging to the primary combustion unit of the rotary kiln; PA1 the oxygen-rich fluid is oxygen with a purity greater than 90% that is passed through said fuel injector via an oxygen-specific passage; PA1 an oxygen-specific passage is located radially on the inside of said fuel injector; PA1 an oxygen-specific passage is located radially on the outside of said fuel injector; PA1 a passage of said fuel injector near to an oxygen-specific passage is used to introduce at least one high-quality fuel so as to form a pilot flame at the outlet of said fuel injector; PA1 at least one fuel flow and at least one air flow are produced in said fuel injector and at least one air flow and/or fuel flow produced in said fuel injector is enriched with oxygen (FIGS. 2 and 4); PA1 a fuel flow is produced in said fuel injector by introducing a fuel and a carrier fluid for this fuel into the fuel injector, and this fuel flow is enriched with oxygen by enriching the carrier fluid with oxygen; PA1 said fuel is introduced into said injector in the form of a fluid; PA1 said fuel is introduced into said injector in the form of solid particles; PA1 the carrier fluid is enriched with oxygen until it has an oxygen concentration that may be as high as 35%; PA1 said fuel is a low-quality fuel; PA1 oxygen is introduced into the fuel-injection zone of the primary combustion unit of the rotary kiln with a flow rate of between 2 and 20 m.sup.3 /h (STP) per MW of theoretical power supplied by a complete combustion of the fuel(s) injected by the primary combustion unit; and PA1 the calcination process is a process for the manufacture of clinker.
2. Description of the Related Art
The manufacture of cement passes through an intermediate stage involving the manufacture of a product known as clinker. Clinker is a product which is obtained by firing ore-based material, particularly clay and limestone. The material, in the form of a powder, may be supplied to a rotary kiln either in dry form (in a dry process) or in the form of a water-based paste (or slurry) (wet process). The composition of the clinker is generally carefully controlled in order to obtain the desired proportions of the various inorganic materials, particularly calcium carbonate, silica, alumina, iron oxide and magnesium carbonate. After being placed in a kiln, the material that is the precursor to the manufacture of clinker is first of all dried out and heated. Next, this material undergoes calcination in which the carbonates of the various minerals are converted into the oxides of these minerals by the removal of carbon dioxide. While the temperatures are still high, the minerals thus obtained react chemically with each other to essentially produce calcium silicates and calcium aluminates. This last process is known as the clinkering process and takes place in the hot zone of a rotary kiln. The resulting clinker is then cooled and ground then mixed with additional ingredients to form a cement such as portland cement.
The clinker-manufacturing process has, in the past, been performed in rotary kilns which typically have diameters of 3 to 5 m and lengths of 60 to 200 m. Improvements to the process have been made by decarburizing or calcining a variable fraction of the raw meal in a stage in the process preceding the rotary kiln, allowing the use of shorter and more thermally efficient rotary kilns. A process stage such as this may be performed in preheater towers (or suspension preheaters), in LEPOL grates or in flash calcination devices.
The extent to which the raw meal is decarburized before it enters the rotary kiln is typically 10 to 45% in the case of suspension preheaters and LEPOL grates and 90 to 95% in the case of flash calcination devices. The energy required for the highly endothermic decarburization stage is supplied by introducing a fraction of fuel into the calcination zone.
Thus, processes for the manufacture of clinker are generally carried out in plants which comprise, in succession:
Types of precalcination device other than those mentioned hereinabove may be calcination chambers or devices known by the name of riser ducts.
In all that follows, the terms "upstream" and "downstream" are to be understood as being with respect to the direction in which the material in such a plant flows.
One or more burners are arranged at the downstream end of the rotary kiln to supply the calorific energy needed for this kiln to operate. The flue gases produced by the burners downstream of the rotary kiln flow against the flow of the material in the plant and supply some of the calorific energy needed for the operation of the precalcination device. Additional energy is provided to this precalcination device by one or more burners.
In general, the search is on to limit the cost of the manufacture of clinker and to improve the processes used for the manufacture of clinker.
Hence, documents U.S. Pat. Nos. 5,572,938 and 5,580,237 relate to the burners downstream of the rotary kilns and propose that the injectors of these burners be modified so that oxygen-injection lances can be introduced thereto. The solutions described in these documents make it possible, with high-quality fuel, to improve the production efficiencies and/or reduce the production of pollutants.
However, these solutions still lead to the emission of relatively significant pollutants.
Elsewhere, the search is on to use low-quality fuels for supplying the calorific energy needed for the operation of clinker-manufacturing plants.
Low-quality fuels are to be understood as meaning fuels which have net calorific values (NCVs) lower than 15 MJ/kg, or water content by mass in excess of 20%. This category also covers fuels which contain less than 20% by mass of volatilizable substances or substances which cannot be reduced to small-sized particles or droplets. In respect of this last criterion, a fuel thus reduced, in which the proportion by mass of particles or droplets of a size exceeding 200 .mu.m is greater than 75%, is considered as being a low-quality fuel.
Industrial waste, such as waste water or solid waste, for example of plastics or cardboard, constitutes low-quality fuels that can be used in the manufacture of clinker.
Clinker manufacturers are looking to increase their consumption of low-quality fuels given their very low costs, these manufacturers even sometimes being paid to incinerate industrial waste such as waste water.
However, the use of such fuels in large quantity poses problems because the flames produced with these fuels are unable to meet the thermal constraints required in the correct implementation of clinker-manufacturing processes.
The object of the invention is to solve these various problems by providing a process for calcining an ore-based material which, in particular, allows the manufacture of clinker at low cost, particularly using low-quality fuels, while at the same time limiting the emission of pollutants.